This invention relates to pressure relief by-pass control valves, especially as used with heat exchangers in oil cooling and like systems. In such systems, should oil flow through the heat exchanger core be unduly restricted or obstructed the pressure relief valve opens a core by-passing route of flow. By this means a continuing flow of oil is assured, despite a blocked heat exchanger core, even though the oil is uncooled, or only partly cooled, and not in its best condition for use. Also, a build-up of very high, potentially damaging, oil pressure is avoided.
In some arts, as in the field of aerospace, the course of development of heat exchangers has been in the direction of giving these devices compact, high performance characteristics with the view of putting the greatest amount of heat transfer surface in the smallest possible space. Heat exchanger cores are made to a very dense construction with only narrow sometimes circuituous passages provided for flow of the confined oil or other fluid to be cooled. A maximum permitted oil pressure drop across the core is set by design specifications and at or about that pressure differential level the pressure relief valve opens. In normal circumstances, the maximum permitted pressure drop is exceeded under cold, start-up conditions when oil viscosity is high. The pressure relief valve opens under these conditions but recloses when rising oil temperatures and lowered oil viscosities reduce the resistance to flow through the narrow passages of the core. An abnormal flow condition arises when narrow core passages become blocked, as by foreign material entrained with the oil or by a build-up of substances on the heat transfer surface drawn from the oil itself. As this clogging condition develops, the pressure drop across the core rises and the pressure relief valve opens, placing the system in a constant by-pass mode.
It is desirable that some means be provided to signal the fact of an open by-pass valve. Operating entities supplied with or using the flowing oil may have a limited functioning time using uncooled oil. It is further desirable, however, that normal valve opening movements, as during a system warm-up operation, may not be recognized as signalling a malfunction.
Heat exchangers with pressure relief by-pass control valves are commonly known and used in the prior art. I am not aware of valves of this kind adapted to signal the fact of a blocked heat exchanger core, nor am I further aware of signalling mechanisms capable of distinguishing between valve movement caused by a blocked heat exchanger core and valve movement occurring normally, as in warm-up operation.
I am aware of prior art filtering systems where a clogged filter is by-passed, with an indicating mechanism being activated to signal a switch to the by-pass mode. Examples of these syssystems are found in U.S. Pat. Nos.:
______________________________________ Hammer et al 3,651,827 3-28-72 Worwood et al 4,498,495 2-12-85 Barnard, Jr. 4,574,836 3-11-86 Chen 4,654,140 3-31-87 ______________________________________ p All of the reference teaching are concerned with magnetic activation of an indicating mechanism. Only one thereof, namely Chen, takes up the problem of suppressing false signals, that is, signals occurring as a function of low temperature--high viscosity conditions rather than in consequence of a blocked filter. Chen proposes to solve this problem through use of a lock-out principle of operation. A magnet controlled indicator button is held by a bimetal arm from being projected to a signalling position until fluid temperatures reach normal operating levels.
Thermostatically controlled by-pass valves also are known, including De Salve U.S. Pat. No. 4,469,275, dated 9-4-84 showing a "pop-up" indicator signalling a failed thermostat.